1. Field of the Invention
The invention relates to radar systems and more particularly to a radar system utilizing a spread spectrum and a combination of coherent and non-coherent signal processing techniques.
2. Description of the Prior Art
A conventional radar operates by transmitting a pulse modulated sine wave signal, detects returned pulses from reflecting objects in the transmission path, and determines the time interval between the transmission and reception of these signals. Maximum range for these radars is a function of the total energy transmitted with each pulse while range resolution capability is a function of the pulse width, increasing with decreasing pulse widths. Since the energy within each pulse is equal to the product of the peak power times the duration of the pulse, it is evident that a trade-off between maximum range and target resolution exists for radar systems that are peak power limited. The long pulse necessary to achieve long range capability for these radar systems need not, however, be incompatible with good range resolution.
It is well known that radar range resolution is a function of the radar signal bandwidth, the resolving capability improving with increasing radar signal bandwidth. In a pulsed monochromatic system this bandwidth is substantially equal to the inverse of the pulse width, thus decreasing with increasing pulse width. Compensation for this reduction in bandwidth with increased pulse width may be realized by providing a frequency modulated carrier within the pulse. By appropriately processing the pulsed frequency modulated signal returned from a target the pulse may be compressed to form a pulse of a width corresponding to the transmitted bandwidth, thus providing a resolution capability which exceeds that achievable with a pulsed single frequency signal having the same pulse width. Maximum range for these pulse compression systems, as in the conventional single frequency pulse modulated systems, is a function of the total energy transmitted within each pulse and, for a peak power limited system is increased with increased pulse width. Since the receiver is generally disabled during transmission, this lengthening of the transmitted pulse adversely affects the minimum range of the system and may necessitate the utilization of a conventional pulsed radar system for coverage to the minimum range of the pulse compression system. Additionally, extremely high resolution and very long range operational characteristics necessitate very large time bandwidth products which require complex circuitry, are difficult to achieve, and are expensive to implement.